Running of a motor by an inverter is normally performed with a fixed ratio of V (voltage)/f(frequency). If a running frequency has been determined, the motor has a slip at which it exhibits the highest efficiency , and this value does not depend on the magnitude of the load.
In running wherein ratio V/f is fixed, if frequency f is determined, then since the voltage is fixed, as the load varies, the slip varies and accordingly the efficiency varies, and therefore, it is impossible to always run the motor at the highest efficiency. Here, several conventional techniques of the type described are slightly examined.
As conventional technique 1, Japanese Patent Publication No. 41029/1991 discloses a method for reducing the drop in efficiency caused by load variation. According to conventional technique 1, a running method for an induction motor driven by a voltage type inverter is characterized in that, when the motor enters a steady running condition, an output voltage and an output current of the inverter are multiplied to determine output power P.sub.N and the output voltage of the inverter is reduced to voltage V.sub.1 which is lower by a V than output voltage V.sub.0 at the beginning of the steady running. The difference .DELTA.P.sub.1 between output power P.sub.1 of the inverter calculated at that time and output power P.sub.0 of the inverter in the beginning of the steady running is compared with constant .DELTA.P.sub.S and then, if .DELTA.P.sub.1 &lt;.DELTA.P.sub.S, the inverter output voltage is fixed to the voltage V.sub.1. But if .DELTA.P.sub.1 .gtoreq..DELTA.P.sub.S, and after a fixed period the inverter output voltage is further reduced by .DELTA.V from voltage V.sub.1, the value of the reduction (difference) of the output power at that time and the constant .DELTA.P.sub.S are compared with each other and the inverter output voltage is fixed or further reduced depending upon the relationship in magnitude between them, the steps being repeated to run the motor with the output voltage of the inverter.
Further, as conventional technique 2, Japanese Patent Laid-Open No. 203700/1995 discloses a method wherein a slip to obtain the highest efficiency is calculated from the motor constant and the frequency. The application of voltage to the motor is controlled to obtain the calculated slip. By this method, when the motor enters a steady running condition, the output voltage and the output current of the inverter are multiplied to determine output power P.sub.N, and the voltage which will achieve the highest efficiency with this power and frequency is calculated and supplied to the motor to control the motor indirectly for optimum slip.